In late years, mobile communications devices have become widespread, and frequency bands up to about 2 GHz band have come to be used in mobile communications. In a base station for communications in such frequency bands, a dielectric waveguide filter has been used which comprises a combination of a plurality of resonators each composed of a dielectric waveguide.
The dielectric waveguide filter is capable of facilitating downsizing and weight reduction based on a wavelength shortening effect of dielectric, and thereby can be directly mounted on a printed circuit board.
However, the dielectric waveguide, and a strip line used in the printed circuit board, are different from each other in terms of a transmission mode of electromagnetic wave. Thus, as a prerequisite to using the dielectric waveguide filter while being directly mounted on the printed circuit board, it is necessary to provide, between the strip line and the dielectric waveguide, an input/output coupling structure for performing mode conversion.